1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Related Art
High-performance LSIs have been developed through miniaturization of transistors. In a currently available LSI, the thickness of the gate oxide film is almost as small as 1.5 nm. According to ITRS (International Technology Roadmap for Semiconductors), the thickness of each gate oxide film is expected to be approximately 0.7 nm by the year 2010, if the miniaturization progresses for even higher performances. In a conventional silicon oxide film with such a small thickness, a direct tunneling current that does not depend on voltage is dominant. Therefore, it is difficult to control the leakage current of the gate oxide film by adjusting voltage, and the performance as an insulator cannot be expected. To counter this problem, a material (a high-permittivity (high-k) material) with a higher relative permittivity than that of a silicon oxide film needs to be employed so as to obtain a greater physical film thickness.
Conventionally, SiON films have been used in place of silicon oxide films. A SiON film is formed by adding nitrogen to a silicon oxide film, so as to obtain a higher relative permittivity. A semiconductor device that has a SiON film on the surface side and a silicon oxide film between the SiON film and the substrate has been known (disclosed in JP-A 2003-264190 (KOKAI), for example). In such a semiconductor device, the SiON film is formed by nitriding the surface and the neighboring region of the silicon oxide film. Since the relative permittivity is increased by adding nitrogen to the silicon oxide film in the semiconductor device, the physical film thickness can be reduced, while the interface characteristics of the silicon oxide film are maintained. Accordingly, the leakage current can be reduced. In this manner, the structure of the silicon oxide film at the interface is maintained, and the nitrogen concentration in the SiON film is increased, so that the leakage current of the SiON film is reduced while the interface characteristics are maintained.
However, the high nitrogen concentration causes the problem of a significant shift in flat-band voltage. As the nitrogen concentration increases, the flat-band voltage shifts. Therefore, in terms of design, it is difficult to increase the nitrogen concentration.
As described above, so as to obtain a SiON film with a higher permittivity and a smaller film thickness, it is necessary to develop a process for allowing a higher nitrogen concentration. However, a SiON film characteristically exhibits a greater shift in flat-band voltage as the nitrogen concentration increases.
We found out that there are two causes for this phenomenon. One of the causes is the formation of defects such as interstitial silicon and dangling bonds due to the introduction of nitrogen. These defects remain in the form of fixed charges in the film, as the number of nitrogen atoms increases. As a result, a greater shift is caused in the flat-band voltage, as the nitrogen concentration becomes higher. The other cause is the boron atoms that are scattered from the gate electrode. The boron atoms are bonded to nitrogen atoms, resulting in silicon dangling bonds. With the a priori cause and the a posteriori cause, the flat-band voltage of the SiON film shifts as the nitrogen concentration becomes higher.
Therefore, so as to further increase the nitrogen concentration in the SiON film, formation of defects due to nitrogen introduction, and bonding between nitrogen and boron entering the SiON film should be prevented. The inventors have already developed a method for preventing the bonding between nitrogen and boron, and have filed a patent application (Japanese Patent Application No. 2005-30586) concerning the method. However, any method for preventing the generation of defects has not been developed yet.